The average fitness ("selective value") of a number of wild-type strains of Drosophila melanogaster will be estimated from the intra-specific competitive ability of these strains in competition with a standard strain carrying compound autosomes. Strains to be studied include newly collected strains from nature, inbred strains, and laboratory wild-type strains. At the same time, the interspecific competitive ability of these strains will be studied, along with their equilibrium biomass and population size. The major focus of the research is to establish whether the various measures of population fitness are highly correlated. From these experiments we will also learn whether natural populations differ in selective value.